High voltage electrolytic capacitors are employed as energy storage reservoirs in many applications. These capacitors are required to have a high energy density because it is desirable to minimize the overall size of the device. Metal foils (e.g., aluminum foil) are often used in high voltage electrolytic capacitors due to their small size. Because the electrostatic capacitance of the capacitor is proportional to its electrode area, the surface of the metallic foil may be, prior to the formation of the dielectric film, roughened or subjected to a chemical conversion to increase its effective area. The capacitance of the electrolytic capacitor is determined by the extent of roughing (the surface area) of the anode foil and the thickness and the dielectric constant of the oxide film. Due to the limited surface area that may be provided by etching metallic foils, attempts have also been made to employ porous sintered pellets in wet electrolytic capacitors—i.e., “wet tantalum” capacitors. A tantalum pellet, for instance, may be formed by compressing a powder under high pressure and sintering at high temperature to form a sponge-like structure, which is very strong and dense but also highly porous. The porosity of the resulting tantalum pellet provides a large internal surface area. Despite its high surface area, however, tantalum pellets may still present high equivalent series resistance (“ESR”) and sensitivity of the capacitance to frequency, particularly at high voltages.
As such, a need currently exists for an improved wet electrolytic capacitor.